One way to achieve good adhesion of plated deposits on passivated metal substrates is by chemical activation, that is by removal of a passivating film by chemical means, such as by acids, prior to plating the surface. However, the success of this method depends on the speed at which repassivation will occur in relation to the time interval between activation and plating. Furthermore, the use of chemical activation may result in contamination of the next plating bath with drag-in of ingredients of the activating bath.
Strike plating may be used as an alternative way of achieving good adhesion of plated deposits on metal substrates to be plated. Strike plating is a deposition of a very thin film of fine nuclei of a selected metal over the surface of a metal substrate to be plated. Strike plating serves to improve adhesion of an electrodeposit on a substrate, especially on a passivated substrate, to protect the main plating bath from contamination by corrosion products of the metal substrate, and to diminish drag-in from previous operations. It can also lead to a reduction in the porosity of subsequent plating coatings, especially of thinner coatings (i.e., &lt;50 microinches). Therefore, the strike plating seems to be a more advantageous alternative.
Strike baths adapted for different surfaces and platings are in commercial use throughout the electroplating industry. For instance, highly acidic nickel strike baths (e.g., Wood's nickel) are used on nickel, stainless steel and cobalt alloys, among others; acid gold strike is used as a preplate on nickel and other substrates before gold or other precious metals, including palladium and platinum; silver strike is used before silver plating; copper strike baths have many applications ranging from lead and beryllium alloy to low-carbon and stainless steel substrates for better adhesion, and on zinc and zincated metals for corrosion protection.
Acid palladium electroplating baths apparently found no commercial use. Highly acidic palladium baths attack the substrate and may cause metal displacement, which is undesirable. In the pH range of from 2 to 7, palladium baths lead to co-deposition of hydrogen with possible cracking of the deposits.
Often the strike and the subsequent plating are not of the same metal; however, bonding is better when the deposit has the same or a similar crystallographic structure, which allows epitaxial growth. Therefore, it is desirable to provide a palladium strike chemistry for plating of palladium and palladium alloys on metal surfaces other than palladium and palladium alloys.
U.S. Pat. No. 4,098,656 issued on Jul. 4, 1978 to John Martin Deuber discloses a palladium bath containing Pd as Pd(NH.sub.3).sub.2 Cl.sub.2, EDTA and two brighteners (Class I-unsaturated sulfonic compounds and Class II-unsaturated or carbonyl organic compounds), the bath having a pH value from 4.5 to 12. It is suggested that the bath with palladium content of from 0.1 to 5 g/l and with pH of 4.5-7, preferably 6.5, could be used for strike plating. However, it seems that this bath has also found no commercial use. This may be explained by its being not suitable for in-line plating because it decomposes easily. While freshly prepared baths are stable, during the plating operation EDTA undergoes oxidation and/or reduction at the electrodes with formation of compounds which can reduce Pd in the solution and subsequent precipitation of Pd from the solution.
Therefore, there is still a need for an acid palladium strike bath which could remove or at least reduce the passivation of a surface to be plated and to present a surface readily plateable by subsequent palladium or palladium alloy baths, and which results in better adhesion than chemical activation (acid pickle).